Field of the Invention
This invention relates to locks for securing doors and cabinet openings and, more particularly, to cam locks that are used in combination with plastic or polymer doors and cabinets.
Description of Related Art
Cam locks are recognized in the prior art as devices that secure doors, drawers, cabinet tops, and the like from unauthorized opening. In general, a cam lock includes a lock body having an internal spindle that is rotatable, and a cam (latch arm) that extends radially from the inner end of the spindle to engage a strike or a fixed structural component. A key is insertable into the lock body to rotate the spindle from a closed position in which the cam engages the strike or structural component, to an open position in which the cam is free of any engagement.
Cam locks are often used in low security situations in which the intent is to prevent casual opening of a cabinet door or drawer or the like. In these situations the lock body may incorporate a simple mechanism that accepts only a certain type or shape of key, but it does not use extensive tumbler locking arrangements.
A typical cam lock in the prior art extends through a hole in the item (door, drawer, panel) being secured. The hole is often standardized as a “Double D” hole, a partially circular hole having opposed parallel flats, and the hole is placed adjacent to a structural frame component or latch strike. The cam lock body is likewise provided with a primarily cylindrical shape with the inclusion of parallel flats that are complementary to the hole shape. The exterior end of the lock body includes a radial flange that impinges on the outer surface of the item being secured, and the lock body is typically secured by a nut secured on threads at the interior end of the lock body, or occasionally a spring clip. The flat surfaces of the lock body engage the parallel flats of the double D hole so the lock cannot turn in the hole, and the nut may be well-tightened to secure the lock.
The components enumerated above are typically fabricated of metals such as steel, brass, and the like, and are compatible with the items being secured, which are likewise fabricated of metal. However, in recent times cabinets, doors, drawers, tops, and similar items to be secured more often have been fabricated of plastic or polymer materials. It has been observed that the metal components of a typical cam lock are not necessarily compatible with plastic or polymer items to be secured, or vice versa. For example, if a metal cam lock is secured to a plastic closure by a nut or a spring clip, the compression of the nut or clip combined with the thermal expansion/contraction of the plastic may degrade the plastic components over time, due to the metal being substantially harder and less thermally active than the plastic material. As a result, degradation of the plastic components increases with continued use, causing failure of the plastic. Also, the double D hole is difficult to form in plastic, so a simpler choice is to drill out a round hole that does not grab the flat sides of the lock, which then tends to turn in the hole. This tendency to rotation of the lock causes the installer to tighten the nut even more.